1. Field of the Invention
The present invention relates to a sintered metallic alloy suitable for an internal gear such as a planetary gear, a method of manufacturing the sintered metallic alloy, and a sintered alloy gear employing the sintered metallic alloy.
2. Description of the Related Art
Regarding such a kind of conventional sintered metallic alloy, a method of manufacturing the sintered metallic alloy, and a sintered alloy gear employing the sintered metallic alloy, a sintered metallic gear manufactured, for example, through manufacturing steps such as those shown in FIG. 12(a) through FIG. 12(g) is known.
That is, metallic materials, such as iron powder, any other metal powder, and a lubricant, for sintered metallic parts such as a sintered internal gear 10, such as those shown in FIG. 12(a), are blended and mixed at a predetermined ratio by a mixer 11, as shown in FIG. 12(b). As shown in FIG. 12(c), a predetermined weight of the mixed metallic materials is poured into a metal mold 12.
Then, an upper punch 12a and a lower punch 12b of the metal mold 12 are slid to move toward each other along the core rod 13, and a pressure of 3 to 7 tonf/cm.sup.2 is applied to the mixed metallic materials from directions above and below, whereby compression molding is performed.
Next, as shown in FIG. 12(d), the compression molded compacts 15 are heated in a sintering furnace 14 for a predetermined time at a high temperature less than the melting point, whereby the diffusion bonding of metal particles in the compacts 15 is promoted to solidify them.
In order to further enhance the quality and characteristics and obtain finished products corresponding to various purposes or uses, after-treatment is performed.
Subsequently, as shown in FIG. 12(e), in order to obtain the sintered internal gear 10, the solidified compact 15 is put into a sizing metal mold 16. An upper punch 16a and a lower punch 16b of this sizing metal mold 16 are slid to move toward each other along the core rod 17, and a pressure is applied to the compact 15 again from directions above and below. By that, sizing is performed.
After sizing, inspection is performed as shown in FIG. 12(f), and a sintered metallic part such as the sintered internal gear 10 is obtained as a finished product (FIG. 12(g)).
An after-treatment technique related to the aforementioned after-treatment is disclosed in Japanese Laid-Open Patent Publication No. SHO 58-19412 (19412/83). The disclosed after-treatment technique is as follows. That is, after sintering in a sintering furnace, quenching is performed at a re-raised temperature of 860.degree. C., and thereafter, tempering is performed at about 180.degree. C., whereby the resistance of the sintered alloy gear to high surface pressure is enhanced.
However, in the sintered alloy gear constructed in the aforementioned manner, the structure in a range of 0.02 to 0.3 mm from the exterior surface toward the inside consists of an austenite layer having conformability, so that due to quenching and subsequent tempering, a thin austenite layer containing austenite entirely different in toughness in high density is formed on the surface of an inner martensite layer containing martensite having high brashiness in high density, and therefore, the inner and outer structures of the sintered alloy gear are separated from each other.
For this reason, the surface of the internal gear including relatively narrow area of contacting surface portion, which high pressure is applied to, is constituted by the aforementioned martensite layer. As a result, toughness is insufficient at microscopic portions and it is difficult to support stress load at a point.